This invention relates generally to shape memory materials, and more particularly relates to shape memory alloy wire composition and production.
Shape memory materials are solid state materials that can undergo a reversible transformation between two distinct morphological phases, namely, a martensitic phase and an austenitic phase. Such phase transformation can in general be induced by exposure to an external stimulus such as, e.g., a change in temperature or an applied mechanical stress, thereby displaying a shape memory capability and a superelasticity capability, respectively. The most widely employed shape memory materials are metals, and in particular metal alloys. Shape memory alloys (SMAs) are well-known for their ability to transform between martensitic and austenitic phases with superior shape memory and superelastic behavior. This phase change behavior enables a very wide range of electromechanical actuation configurations as well as energy dissipation and mechanical damping. As a result, SMA materials are important for many advanced engineering applications.
Many advanced applications for SMA materials require microscale mechanical configurations of the SMA into a selected geometry. But the microscale counterpart to macroscale SMA structures such as ribbons, plates, and wires are technically very challenging to achieve. Specifically, the production of micro-scale structures of shape memory alloys remains a nontrivial materials processing challenge. Because shape memory alloys tend to undergo a stress-induced martensitic transformation, deformation processing of shape memory alloy materials in the formation of a microstructure can be problematic; the materials retain a memory of the unprocessed, undeformed shape. Further, conventional SMA materials such as Cu—Al—Ni and Cu—Zn—Al exhibit poor cold-workability due to their high-degree order in the parent phase with B2, D03, or L21 structure as well as an extremely high elastic anisotropy ratio in the β phase.
For example, there has been shown the production of shape memory alloy wire, and in particular copper-based SMA wire, by a process including hot rolling followed by cold rolling. But this dual-rolling production technique is limited to formation of relatively large wire diameter, e.g., greater than 500 μm, due to the limited workability of the SMA material. To overcome this limitation, it has been shown to codraw a SMA composition in the liquid phase within an outer glass capillary. This drawing technique overcomes the limitations of the mechanical rolling process, but requires a post-production step of glass layer removal to uncover the drawn wire and cannot continuously produce long lengths of wire.
Indeed, it is found that microscale production of SMA material structures remains difficult, and for many applications, cost-prohibitive, inflexible, and unable to be adapted for continuous processing. As a result, advanced technical applications requiring SMA microscale structures such as SMA fibers cannot be optimally addressed.